The invention relates generally to the field of decolorizing of diamonds. More specifically, the invention relates to a method for decolorizing or altering the color of diamonds near or below atmospheric pressure.
A diamond is a crystalline allotrope of carbon. Diamonds are broadly classified into natural and synthetic diamonds, on the basis of their origin. A natural diamond is a diamond that is formed naturally in the earth by prolonged exposure of carbon-bearing materials to high pressure and temperature. Scientists have been able to produce synthetic diamonds in laboratory conditions, which have the same chemical composition and physical properties as natural diamonds.
Natural diamonds possess a color that ranges from clear and colorless to yellow, orange, red, pink, blue, green, brown, and even black. The four ‘Cs’, i.e., cut, color, clarity and carat size, generally determine the price of a diamond. However, the price of the diamond largely depends on the carat size and color of the diamond. In natural diamonds, brown-colored diamonds are most common and relatively inexpensive. Fancy-colored diamonds, and clear or colorless diamonds are highly priced, and are extensively used in making jewelry. Even the faintest tinge of yellow in colorless diamonds considerably reduces the price of the colorless diamonds.
Therefore, diamonds are decolorized to increase their aesthetic appeal and economic value. Low priced brown-colored diamonds are decolorized to colorless to increase their economic value.
Diamonds have been decolorized, until now, by using a High Pressure High Temperature (HPHT) process. In the HPHT process for decolorizing diamonds, the diamonds are placed in an HPHT press and exposed to high temperatures (mainly above 1700° C.), and high pressures (a few gigapascals). The diamonds in the HPHT process are generally maintained in the thermodynamic pressure-temperature (P-T) stability region of the diamond by applying sufficient pressure. This is done in order to prevent graphitization of the diamonds, which results in a re-arrangement of carbon atoms in the diamonds leading to a conversion of a diamond into graphite.
At high temperatures and high pressures maintained inside the HPHT press, the mobility of atoms in the diamond increases. The increased mobility of atoms leads to migration of nitrogen atoms present in the diamond (responsible for color of the diamonds) to form aggregates. The formation of aggregates of nitrogen from the nitrogen atoms contributes to the decolorizing of diamonds.
U.S. Pat. No. 4,399,364, titled ‘Diamond Treatment’ describes an HPHT method for reducing the color of type 1b diamonds. The method involves irradiating the type 1b diamonds by electron or neutron bombardment leading to atomic displacements in the diamond crystal. This is followed by heat-treating the irradiated diamonds at a temperature in the range of 1600° C.-2000° C. The diamond is heat treated in an HPHT apparatus at 80 Kbars. The period of heat treatment varies according to the degree of the desired reduction in the color.
U.S. Patent Publication Number US20020172638, titled ‘High Pressure/High Temperature Production of Colorless and Fancy Colored Diamonds’ also describes an HPHT method for improving color of a discolored natural diamond. The discolored natural diamond is heat treated in the elevated temperature range from 1500° C. to 2700° C. at pressures ranging from 5 GPa to 20 GPa. The diamond is heat treated for a time period ranging from 30 seconds to 500 hours.
However, one of the limitations of using HPHT process for decolorizing diamonds is that the HPHT press needs to be operated in the P-T stability region of the diamond. An inability to operate the HPHT press in the P-T stability region of the diamond results in the graphitization of the diamond, which reduces the economic value of the diamond. The outer surface of the diamond gets graphitized even when the HPHT press is operated in the P-T stability region of the diamond. The graphitization of the outer surface of the diamond results in the formation of a black layer on the outer surface of the diamond. Therefore, the diamond decolorized by the HPHT process needs to be repolished. Further, high pressures inside the HPHT press may lead to development of cracks in the diamond and frequent breakages of the diamond, i.e., the diamond is unable to survive high temperatures and pressures for a substantial period of time. Furthermore, the HPHT process for decolorizing diamonds is not very cost-effective, as some parts of the HPHT press suffer damage at high temperatures and pressures maintained inside the HPHT press. As a result, these parts of the HPHT press need to be replaced every time diamonds have to be decolorized. This increases the cost of operating the HPHT process.
In view of the foregoing discussion, there exists a need for a method for decolorizing of diamonds, which is able to produce clear or colorless diamonds without damaging the diamonds or damaging the system used for decolorizing of diamonds. Further, the cost of operating the method should be low. In addition, the method should enable decolorizing of the diamonds at pressures near or below atmospheric pressure in order to prevent development of cracks in the diamonds and breakages of the parts of the system used for decolorizing of diamonds. Further, the method should enable decolorizing of the diamonds without causing the graphitization of the diamonds. Furthermore, the method should prevent the need of repolishing the outer surface of the diamond. In addition, the method for decolorizing should result in uniform decolorization of the diamonds.